While there is a clear inherited basis to rheumatoid arthritis, a destructive arthritis that afflicts up to 1% of the adult population, our genetic understanding remains incomplete. From the mid-1970's until 2007, only two RA susceptibility loci had been confirmed unequivocally; over the last year, we have identified and validated two additional gene loci. However, three key issues are: (1) we do not know the exact causal mutation at each locus, (2) we do not yet have a complete catalogue of all RA mutations, and (3) we do not understand how these alleles perturb human immune cells to cause disease. The goal of this proposal is to address these three key issues. Importantly, these key issues impact the long-term goal of using genetics to predict risk of RA prior to clinical onset and to develop new therapeutics. To accomplish our goals, we proposed three Specific Aims: Specific Aim 1: Identify the common causal mutations two newly identified RA susceptibility gene loci. We have recently identified two new RA susceptibility loci on chromosomes 9q33 (includes TRAF1 the gene) and 6q23 (includes TNFAIP3). We will use state-of-the-art sequencing technology (Solexa) to identify all known genetic variants within these loci, and define the most accurate set of putative causal mutations at each locus in more than 5,000 RA case-control samples. Specific Aim 2: Search for rare protein coding mutations in the TRAF1 and TNFAIP3 genes that also contribute to RA risk (independent of common mutations). We will again use state- of-the-art sequencing technology (Solexa). Here, we will search for rare DNA variants that are more commonly observed in RA cases than in controls by re-sequencing the coding regions of TRAF1 and TNFAIP3 in 500 RA cases and 500 RA controls. Specific Aim 3: Test the function of common and rare RA susceptibility mutations using hypothesis-driven experiments. We will generate hypotheses about the predicted function of common and rare variants (Aims 1 and 2), and test these hypotheses in a relevant immune tissue type (e.g., mononuclear and B-cells). This will provide insight into how these alleles cause RA. Completing these Aims will provide substantial progress towards our ultimate goal of a complete understanding of all RA genetic mutations - a necessary step before genetics can be translated to clinical care. Moreover, this approach will be applicable to other RA susceptibility genes as they are identified.